JOURNALOF GEOPHYSICALRESEARCH
VOLUME 66, No. 1
JANUARY 1961
Helium, Argon, and Carbonin SomeNatural Gases
R. E. ZARTMAN AND G. J. WASSERBURG
The Cali/ornia Institute o• Technology
AND
J. H. REYNOLDS
University o.f Cali/ornia
Berkeley, California
Abstract. Thirty-nine samplesof natural gasesrepresentingvaried chemicalcompositionsand
geologicaloccurrences
were analyzed for their helium, radiogenicargon, and atmosphericargon
contents.The total range in the (He/A)r•d ratio was found to be 1.6 to 130 with most samples
having values between6 and 25. This range of valuesis essentiallyequal to the productionratio
from the uranium, thorium, and potassiumin averageigneousrocks and a wide variety of sediments. This indicates that all of these natural gaseshave obtained their radiogenic gasesfrom
rather averagerock types. This is true in spite of the fact that the gasesrangein helium content
from 37 to 62,000 ppm.
A theoretical discussionof the origin of helium and argon in natural gasesis given. It can be
shownfrom the ratio of nitrogen to atmosphericargon that most of the nitrogen in these gases
cannotcomefrom the entrapment of air. From a considerationof the concentrationof atmospheric
argon in natural gasesit is possibleto estimate the proportion of gaseousand aqueousphases
assumingdiffusive equilibrium.
The isotopiccompositionof the carbonin the methane of thesegaseswas found to be very light.
It was shownthat for coexistingCH 4-CO 2pairs the carbondioxidewas alwaysisotopicallyheavier.
reservoirgreatly enrichedin U and/or Th,
INTRODUCTION
The purposeof this study was to investigate
the relationship between the abundances of
helium and argon in natural gasesof different
compositions
and environmentsand to examine
the isotopiccompositionof the carbon in these
gases.Sincethe discoveryof terrestrial helium
in natural gasesby Cady and McFarland [1906],
a vigoroussearchhas beenmadefor helium-rich
gases,and a considerablenumber of total gas
analyses that include helium determinations
or
the
accumulation
from
a
rather
normal
rock reservoir.Lastly, there existsthe possibility
that the helium representssomemore primordial
gases trapped in the earth during its early
history and subsequentlypartly releasedinto
stratigraphicand structuraltraps. Thesevarious
possibilitieshave been recognizedby some of
the earliest investigatorsin the field [Rogers,
1921]. It has been found [Faul, Gott, Manger,
Mytton, and Sakakura,1954;Sakakura,Lindberg,
and Faul, 1959] that severalhelium wells, i.e.,
exist in the literature. These have been made
using both volumetric and mass spectrometric wells containingover 0.5 per cent helium, are
techniques[Rogers,1921; Andersonand Hinson, radioactive owing to a high radon content and
appear to be associatedwith some uraniferous
1951; Boone,1958].
petroleum
residues. Other high helium wells,
The origin of high helium natural gases,some
of which have helium contents as great as
however, are devoid of such radioactivity. In
addition, many well gaseshave a high radon
contentand only smallconcentrations
of helium
[Satterlyand McLennan, 1918].The composition
of helium-rich gaseshas been found to be quite
daughter products.A fundamentalquestion variable,althoughin somegasfieldsa correlation
ariseswhether these quantities of helium repre- has been suggestedbetweenhelium and nitrogen
10 per cent,has beenthe subjectof considerable
speculation. The He4 contained in them is
presumablythe productof the radioactivedecay
of U •'35,U•'% and Th 2•'-,and their intermediate
sent thc accumulatcd dccay products of a
277
content.
278
ZARTMAN,
WASSERBURG, AND REYNOLDS
The isotopiccompositionof helium from gas estimates of the contribution of 'atmospheric
gases.
wellshas been investigatedby Aldrich and Ni•
[1948]who reportvaluesof He•/He4 -- 10-7.
The results of these authors representthe only
EXPERIMENTAL TECHNIQUES
We have studied a variety of natural gases
publisheddata on He• content in terrestrial
gases.The origin of the He• in thesegaseshas coveringa wide compositionaland environmental
been consideredby severalworkers.Hill [1941] range in order to attempt some understanding
suggested
that this isotopeof helium could be of their origin. In this study we have restricted
our effortsto well gasesand have not investigated
producedin rocksby the reactionLi6(n,a)H8-•
He• -]- /•-. More recently, Morrison and Pine volcanic or hot spring emanations.Thirty-nine
[1955]have discussed
the relative productionof samples were analyzed for their helium and
I-Ie• and I-Ie• in rocks. As shown by Wetherill argon contents and for isotopic composition.
[1953, 1954], the principal sourcesof neutrons Partial gasanalysesfor other major constituents
are the reaction O•s(a, n)Ne• and spontaneous were also made. The isotopiccompositionof the
fissionof U TM.By considering
the variousnuclear total gas carbon, methane carbon, and carbon
reactionswhich competefor neutrons,Morrison dioxide carbon were determined. The results of
and Pine concludethat the He•/He• ratio this investigation are presentedin Tables 1 and
observedby Aldrich and Nier in natural gases 2, together with pertinent well data for each
is mostreasonablythe productof theseprocesses sample. The accompanyingtotal gas analyses
in materials which contain neither uranium nor
representdata suppliedus by the participating
thorium, or both, in very great concentration. petroleumand natural gascompanies,partial gas
Their argumentswould not, however,preclude analysesperformed in our laboratory, and a
the originof thisheliumto be fromfinelydivided combination of these two sources. With the
uranium minerals disseminated in a rather
exception of three samples,the H•S contents
normal rock. This would permit uranium con- were less than 0.01 per cent. For samples14,
34, and 35, the H•.S contentswere 0.04, 0.09,
centrationsof up to a few tenths of 1 per cent.
The radioactive decay of K •ø gives rise to and 0.12 per cent, respectively.
With the exceptionof samples29, 30, 31, and
the possibilityof high argon-containing
natural
gases.Sincepotassiumis a principalelementin 32, the well gaseswere collectedusing standard
most crustal rocks as comparedwith uranium high-pressure,stainlesssteel gas cylinderswith
and thorium, a study of the ratio of radiogenic valves on both ends. The cylinders were conheliumto radiogenicargon,(He•/A'ø)•, gives nected to the gas sourceand purged of air by
more direct information on the possiblesource passingwell gas through them under positive
pressurefor several minutes. The outlet valve
of helium gaswells.•
Someresultsusingthis approachwerereported was then closed,and the pressurein the cylinder
by Wasserburg,
Czamanske,
Faul, and Hayden was allowed to reach a satisfactoryvalue (20[1957] for some helium wells in the Texas 3000 psig), and then the valve to the sourcewas
Panhandle. These workers showed that the closed. All of these sample vesselswere at a
over I atmospherewhen
argonin four heliumwellswasabout 70 per cent pressureof considerably
radiogenic
and that the ratio (He4/A•9•.awas they wereusedfor analysis.Samples29-32 from
about 10. This ratio was well within the values the Texas Panhandlegas field were obtained by
to be expectedfrom the present-dayproduction Henry Faul in 1954. They were collected in
rates of He• and A•ø in normal igneousrocks. glasscylindersthat had openingsat both ends.
They therefore concludedthat these helium After allowingthe gasto passthroughthe cylinwellswereprobablyformedfromthe accumulated dersat slightpositivepressurefor a few minutes,
radiogenicgasesfound in rockswith a ratio of both endsof the vesselwere sealedoff. In every
U/K typicalof normaligneousrocks,and that instancethe helium, argon,and carbonanalyses
in no way could these gasesbe the result of were done on the samesample.
accumulation from an enriched uranium reservoir.
The gas cylinderswere joined onto a vacuum
For thesesamples
it wasshownthat the A•/A •6 line (Fig. 1) with metal-to-glasscouplingsand
ratio was the sameas that found in atmospheric the systemwas evacuatedwith all the furnaces
argon and it was thereforepossibleto make heatedand outgassedbeforeeachrun. After the
HELIUM,
ARGON, AND CARBON
279
*<---To hicjh vacuum
Manometer
Cold trap
• t•A
Sample
tubeM•c•
Leod
cjaucje
furnace
Toe
==•..•fur
Fig. 1. Vacuumapparatususedfor helium,argon,and carbondeterminationand extraction.
systemwasfoundto be vacuumtight, somegas
was releasedinto a part of the systemof known
volume and the pressuremeasuredon a manometer. For most samples a tracer of Ass was
introduced and the gases mixed. The sample
size rangedfrom 2 to 100 cc STP. For total gas
carbon analyses, all gas was then pumped
directlyinto the combustion
system.For methane
carbonanalyses,liquid N, was placedon a cold
trap and the noncond•nsible
fraction was
continuously pumped into the combustion
systemuntil no gas phaseremained.The combustion procedure used was similar to that
described by Craig [1953]. A CuO furnace,
made of fusedquartz, washeatedto 900-950øC,
and the gases were cycled through it until
complete combustion to CO•. was obtained.
During this combustionperiod, dry ice baths
were placedon a cold trap to collectthe water
produced.After combustion,the dry ice baths
were replaced with liquid nitrogen and the
noncondensibles--including
N2, He, and A--were
transferred by a Toepler pump into another
sectionof the line (of known volume) containing
a Ti furnace,1VfcLeod
gages,and a sampletube
filled with
activated
charcoal.
The
Ti
furnace
was heated to approximately950øCand the gas
sample purified. The amount of this noble gas
residuewasthen measured,usinga McLeod gage.
Liquid N, was next applied to the charcoaltrap
and the condensible fraction quantitatively
absorbed. The pressure was again measured
and attributed to helium. The argon sample
tube
was sealed off from
the line
and mass
spectrometricallyanalyzed. The helium yield
wascheckedby runningsamplesof knownhelium
concentrationand by comparingthe results of
duplicateanalysesin which the combustiontime
and the amountsof gaswere varied by a factor
of 4. The purity was checkedby observingthe
Tesla discharge color. No detectable loss of
helium by diffusionthrough the fused quartz
combustiontubes was observed.Accordingto
the work of Norton [1953] on the diffusion of
helium through fused quartz at various temperatures,it was expectedthat less than I per
cent of the helium would be lost by diffusion
during the combustionprocedure.Blank runs
carried out under conditions similar to those in
which a sample was being analyzed showedno
appreciableintroduction of inert gasesinto the
line.
The argoncontentwas determinedby (1) the
differencebetweenthe volumetricallydetermined
total noble gas residueand the helium content,
(2) a volumetricdeterminationof the condensible
fractionafter a secondpurification(beforewhich
the helium was pumped off and the Ti furnace
outgassed),and (3) isotope dilution using A88
tracers of known content as described by
Wasserburgand Hayden [1955]. The volumetrically determined argon values are generally
higher than the isotope dilution value. The
first method yields argon contentsup to 40 per
cent higher than the isotope dilution value,
whereasthe secondmethod showsdiscrepancies
of up to 15 per cent. The resultsobtainedby the
first method were highly reproducibleover a
long period of time. They were, nevertheless,
frequently in error, presumablyowing to the
280
ZARTMAN,
ß
WASSERBURG, AND REYNOLDS
'
u'D•00c.o
ooooooo
•oooo
•'•'•-•••'••••
o•oo.
r-,,
.•.•.•
••
••••••oooo.•
•••
HELIUM, ARGON, AND CARBON
281
282
ZARTMAN, WASSERBURG,AND REYNOLDS
TABLE
1.
Continued
B. Analytical data
Lithology of
No.
i
2
3
4
5
6
7
8
9
producing
zone CH4%
C•.H6
%
CO•.%
1.60
1.57
0.86
30.97
3.76
9.15
2.65
4.03
2.40
0.11
0.09
0.06
0.09
0.00
0.03
0.02
1.03
1.82
ss
ss
ss
ss
ss
ss
ss
ss
ss
ss
98.16
95.43
95.12
68.91
96.20
90.70
97.32
84.5
90.5
86.7
11.0 •
12
13
14
15
16
17
18
19
20
21
22
ss
ss
lms
ss
sh and ss
sh and ss
ss
ss
lms
ss
lms
82.7
75.1
56.1
96.52
0.0
0.0
82.58
82.4
86.3
91.10
38.0
14.8
23.2
21.4
1.06
0.0
0.0
15.36
14.8
10.0
7.87
10.6
1.0
0.87
22.2
1.48
0.12
0.15
1.98
2.6
3.4
0.94
42.5
23
24
25
26
27
28
29
30
ss
ss
ss
ss
ss
ssand lms
dolo
dolo
86.2
65.3
97.5
95.1
75.7
76.5
73.8
84.0
8.0 (1)
6.7
2.0 (1)
2.8 (1)
23.6 (1)
15.9
9.6 ( 1)
6.4
5.6
24.9
0.50
1.9
0.64
6.5
16.6
8.95
32
dolo
•i:•
15.2
10
11
31
as
34
35
36
37
38
39
40
41
ss
53.5
dolo
90.3•-
dolo
72.2
as
ss
ss
ss
ss
ss
lms reef
lms reef
90.42
0.0
0.0
86.2
88.4
94.9
89.8
72.6
71.1
0.13
2.37
3.96
0.01
0.04
0.11
0.01
10:5
5.3
N•.%
6.6
. ..
0.0
0.0
9.4
10.3
5.4
7.0
24.2
7.9
2.30
36.6
9.7
9.6
0.60
0.55
2.08
1.22
1.8
3.09
2.5
3.6
1.52
0.77
0.1
0.90
99.9
99.8
0.05
0.17
0.25
0.07
2.1
•)'
...
...
•)'
...
616
99.3
99.3
2.29
0.10
0.85
0.85
0.6
5.0
He ppm
47.5
41.2
37
96.6
101
37.6
85
101
63
140
26000
152
151
1350
359
44.5
46.6
348
480
69.4
172
62200
1640
22600
203
1575
8O5
1720
9370
4180
4480
4170
7OOO
232
187
75
158
757
152
593
1670
A ppm
57.4
52.2
67
140
88.2
53.1
125
32
39
118
1400
26.5
42.0
360
81.9
28.0
29.2
57.3
66.8
117
46.1
5630
77.1
1080
6.8
35.8
14.9
376
877
470
482
461
710
79.5
65.4
27
73.4
66.8
17.5
117
418
Includes
CO2;2Includes
C2H6andhighercarbons,
andCO2.
presenceof contaminatinggases. A second
The carbondioxidein the natural gaseswas
purification
as described
in (2) gavevolumetric removedby placingliquid nitrogenon a cold
valuesin relativelygoodagreement
with (3). trap and pumping off the noncondensibles
All samples
except3, 7, 8, 9, 10,and36havebeen throughanothercold trap in order to retain
run by isotopicdilution,and, exceptfor these any CO•.that mightbe lostfrom the first trap.
samples,the valuesfor argoncontentlistedin The remaininggases,which include CO• and
Tablei havebeendetermined
byisotope
dilution. hydrocarbons
lessvolatilethan CH4, werethen
After the noncondensibles
werepumpedout transferredinto a reactionvesselcontaininga
ofthepartofthelinecontaining
theCuOfurnace, saturatedBa(OH)•. solutionwith 80 per cent
the liquidnitrogenbathsthat holdH•.Oand C02 phosphoricacid in a sidearm. The CO•.wasthen
were again replacedby dry ice bathsand the converted
to thecarbonate,
andthehydrocarbons
C02wasallowed
to sublime
intothispart of the were pumpedoff. The carbonatewas reconverted
line.TheCO•.wasthentransferred
intoa sample to CO•.by reactionwith phosphoric
acidand the
tubefor isotopicanalysis.
CO•.transferred
to a vesselwith liquidnitrogen.
HELIUM, ARGON, AND CARBON
TABLE
1.
283
Continued
B. Analytical data
No.
1
2
3
4
Arad
Aair
ppm
ppm
•%
He/A•.•
A4O/A
•
3.69
3.9
53.7
48.3
6.4
7.5
12.9
10.6
316 4. 3
320 4- 3
4.4
63
6.6
0.219 -4-0.018
8.4
3174- 3
0.202':j:'0.015
21.8
3124- 6
0.196'•' 0.007
124
11.2
6
9.47
7
9.3
8
11.2
9
4.8
10
13.6
11 1260
12
15.9
13
27.2
14 270
15
48.2
16
27.6
17
29.0
18
28.2
19
37.9
20
28.1
21
13.4
22 5580
23
62.6
24
969
25
4.2
26
17.7
27
6.0
28 250
43.6
116
21
35
104
140
10.6
14.8
90
33.7
0.4
0.2
29.1
28.9
89
32.7
50
14.5
111
2.6
18.1
8.9
126
17.8
7.4
35.0
12.3
11.5
90.0
60.0
64.8
75.0
58.8
98.4
99.3
49.2
56.7
24.0
29.1
99.0
81.3
89.6
61.8
49.5
40.2
66.5
3.97
9.2
9.0
13.1
10.3
20.6
9.56
5.55
5.00
7.46
1.61
1.61
12.3
12.7
2.47
12.8
11.1
26.2
23.3
48
89
134
6.88
361 4- 2
320 4- 6
456 4- 14
337 4- 2
336 4- 2
2818 4- 140
736 4- 8
841 4- 12
1185 4- 12
720 4- 6
22500 4- 1100
34000 4- 4000
587 4- 3
684 4- 3
389 4- 2
417 4- 2
29100 4- 4000
1570 4- 20
2720 4- 140
775 4- 30
587 4- 5
494 4- 5
883 4- 10
30
31
32
33
34
35
375
359
336
518
75.2
60.7
95
123
125
192
4.3
4.7
79.7
74.5
72.9
72.9
94.5
92.7
11.2
12.5
12.4
13.5
3.09
3.08
1465 4- 15
1160 4- 12
1100 4- 11
1195 4- 11
5300 4- 120
4150 4- 95
0.201 4- 0.014
0.186 4- 0.023
0.198 4- 0.004
...
...
37
38
39
12.5
59.1
12.2
73.4
17.0
88.4
69.7
62.8
12.6
12.8
12.5
355 4- 3
2560 4- 130
960 4- 24
796 4- 10
0.188 4- 0.008
0.204 4- 0.024
0.178 4- 0.018
...
5
29
36
40
41
15.7
A•s/A•
4.64
697
10.0
381
83.6
180
17
60.9
7.7
5.3
43.6
37
5.3
79.3
37.0
91.1
6.15
13.4
7.5
8.09
4.38
334 4- 2
14354- 14
4724- 15
3316 4- 150
The accompanyingwater was then held with a
dry icebath,andthe purifiedCOstransferred
into
0.195 4- 0.015
0.194 4- 0.007
0.201 4- 0.009
0.195 4- 0.007
0.195 4- 0.010
. ..
...
...
...
. ..
...
...
. ..
...
...
...
...
...
...
...
...
. ..
0.197':•' 0.003
0.203':•'0.010
...
(N:/A,•,)
f,,,
V./V,
298
325
137
2500
450
2100
228
1920
706
221
2610
0.68
0.69
0.46
0.49
0.50
0.56
0.51
0.68
25
24
62
55
53
42
51
25
19
67
943
588
2470
439
3000
7500
0.74
0.44
-0.31
0.83
ii'
O. 74
O. 37
0.54
1.00
19
90
45
0
1.00
0
680
900
382
o. 71
o. 65
o. 43
287
8500
3860
2240
1920
1050
720
515
0.84
o. 36
o. 92
0.60
0.97
0.99
0.97
--0.22
942
789
o. 82
0.77
12
16
768
0.77
16
1395
6'•8
'i'1
1170
1224
200
2340
5830
573
973
0.98
0.82
o. 76
O. 97
0.88
0.35
0.29
1.1
12
17
1.6
7
96
130
792
22
29
70
lO
94
5
35
1.6
0.6
1.6
o.oo
analyzed; the argon isotopic compositionis
givenin Table I alongwith the meandeviation
a samplevesseland massanalyzed.
of the analysis.A88/A86 ratios are given for
In someof the carbondioxidewell samples, samples
on whicha run wasmadewithoutusing
hydrocarbons
werenot detected,and therefore a tracer. The total carbon and methane carbon
the CO: was purifiedby simplypumpingoff isotopic analysesare reproducibleto within
the noncondensibles
at liquidnitrogentempera- 0.2 per mil, and the CO: carbonanalysesare
ture. The Farnham Dome samplescontained reproducible
to within I per mil. The isotopic
considerable
amountsof I-I•S, and this was compositionof the argon was determined on
removedby passingthe gas repeatedlyover mass spectrometers at the Californi• Institute
heatedsilverfilings.
of Technologyand the Universityof California
All of the heliumanalyses
and thoseargon at Berkeley. Agreementwas good on several
analysesthat were determinedby isotopic samplesrun by both laboratories.The backdilutionare most probablyaccurateto within groundwasalwaysmuchlowerthan the sample
10 per cent. The heliumwas not isotopicallypeak heights.
ZARTMAN,
284
WASSERBURG, AND REYNOLDS
The carbon was analyzed at the California sediments,air dissolvedin water associatedwith
Institute of Technology on a 6-inch mass the sediments,gasesbrought in at some later
spectrometerin Dr. Epstein's laboratory. All time by circulating ground water, and consampleson which carbonisotopicanalyseswere tamination during sampling and analysis are
made were run as carbon dioxide accordingto all possiblesources.By meansof blank runs and
the proceduredescribedby McKinney, McCrea, repeat analyses, it was possibleto show that
Epstein, Allen, and Urey [1950]. The samples essentiallynone of the atmosphericargon was
were analyzed relative to Caltech Working dueto contaminationfrom laboratoryprocedures.
StandardII, and then convertedto corresponding Samples were run with atmospheric argon
values relative to the Chicago Standard PDB. contentsaslow as 0.2 ppm without any difficulty.
Repeat analyses of samples at different times
•ITROGEN
AND ATMOSPHERIC ARGON
yieldedidenticalA4ø/A
s6 ratios. Inspectionof
s6ratiosshowsthat thesevaluestend
Mass spectrometricanalysesof the extracted the A4o/A
argon samplesshowedthat while the A•0/A• to be rather constant within certain gas fields.
ratios rangedfrom 312 to 34,000, the A•s/A• Althoughsomeof the samplesmay possiblyhave
ratio was practically constant and was, to been contaminated during sample collection,
within experimentalerror, equal to the value of it is our conclusionthat, for most cases,the
atmosphericfractionsrepresentthe accumulated
0.187 found in atmosphericargon.
Each samplewas found to have an A•ø/A• air argon associatedwith these gasesduring
ratio greater than 295.6, the value for air argon their natural evolution.While the composition
[Nier, 1950].Thesegasesappearto be mixturesof and abundanceof atmosphericargonmay have
radiogenicargon (A•a) and atmosphericargon changedthroughgeologictime, we have assumed
(A•). The fraction, e, of the total argon that is .the present-dayvalues. It is conceivablethat
measurements
of the A40/Aa6ratio may prove
radiogenicis given by the expression
useful as a tracer in identifying and studying
gas reservoircharacteristics.
A• - 1296.8
The natural gasesanalyzed contninedamounts
where (Aaø/A•)• is the ratio in the sample.
• of nitrogenvarying from 0.1 to 42.5 per cent.
These parameters are presented in Table 1. Gases composedof essentially 100 per cent
It is not possibleto say under what conditions nitrogen have been reported in the literature,
the atmospheric argon was introduced into but wehavenot beenableto obtainsuchsamples.
the samples.Air bubblestrapped in the original The origin of nitrogenin natural gaseshas been
often discussed[Hoering, 1957; Scalan, thesis,
• The formulas used •o calculate the amounts of
Universityof Arkansas;Rayleigh,1939; Rogers,
radiogenic (Aaø)• and to•al sample (Aa0)• when a 1921; and Zobell, 1952] in terms of (1) the
tracer, t, was used are:
introductionof atmosphericnitrogen, (2) the
releaseof nitrogenby the bacterialdecomposition
e- Ato•a•
=
and
[\A
(A4ø/ASO)s
d-1.2 (1)
_
\A ],\A
where the subscriptst, n, and y refer to the appropriate ratio in the tracer, in normal atmospheric
argon, and in the sample-tracermixture, respectively.
HELIUM,
ARGON, AND CARBON
of nitrogen-bearingcompounds,(3) the release
of nitrogenby the inorganicchemicalbreakdown
of organiccompounds,and (4) the liberation of
inorganic nitrogen from igneous (and metamorphic) rocks.
One of the processescalled upon most frequently to explain the presenceof nitrogen in
well gasesis the incorporationof air in the pore
spaceof variousreservoirrocks.I[ air is included
in the sediments,they will contain not, only
but other atmospheric gases as well. Some
workers have assumedthat when gaseshave a
285
disintegrationof uranium and thorium, and the
radiogenicfraction of the argon by the electroncapture decay of K •ø, we can make certain
theoretical calculations to determine the values to
be expectedfor the radiogenichelium and argon
abundances
and their ratio (R -- (I-Ie/A),•a).
In order to discuss the abundances of helium and
argon observedin natural gases,it is necessary
to consider (1) the variation of He'- and A •øproductionwith time, (2) the natural distribution
of uranium, thorium, and potassiumin rocks
and their distribution among the mineral phases
(N2/A) ratio approximatelyequal to that in present, (3) the efficiencywith which the helium
air, the nitrogenis dueto atmosphericcontamina- and argon can escapefrom crystal lattices and
tion. It was pointedout by Wasserburg,
Czaman- becomeavailable for accumulation,and (4) the
ske, Faul, and Hayden [1957] for certain high processof gas migration and accumulation into
helium gases from the Texas Panhandle that gas reservoirs.The first part of the discussion
whilethe total (N2/A) ratiosof thesegaseswere will be devoted principally to estimatesof the
approximately equal to the atmosphericvalue, (He/A)r,• ratio. The problem of absolute
most of the argon was of radiogenicorigin, and concentrations and amounts of these noble
the actual (N•/A•ir) ratio was about 850. gaseswill be treated later.
They concluded, therefore, that most of the
Becausethe half-livesof the uranium, thorium,
nitrogen could not be due to trapped air. Fol- and potassiumisotopesare of the order of 10•
lowing this approach,we have made a similar years or more, the rates of productionof helium
calculationof the (N2/A,i r) ratio for our sam- and argon are rather constant for the past
ples. The results are given in Table 1. The several hundred million years. For such a time
ratio is observedto rangebetween137 and 8500. period we obtain the relationships
The atmosphericratio of nitrogen to argon is
.20 U -{- 0.29 Th)• X 10-•
84, and the ratio for the dissolvedgasesin water
in equilibrium with the atmosphere at 20øC
N^ -----3.99 K • X 10-•
is 38. Althoughit may be reasonableto postulate
simple mechanismsof gas solution and effer- where Nu. and N• are the cc STP of helium
vescence, which could account for a factor of and argon, respectively, produced by radio2 or 3 changein the atmospheric
(N,/A, i r) active decay during a time interval of • years•
ratio, most of the gassamplesshowmuchhigher and U, Th, and I• are the weightsof uranium,
nitrogen enrichments.Therefore, in most in- thorium, and potassium in grams. Dividing
stances only a small fraction of the nitrogen these equationswe see that the ratio R of the
content of thesegasesreasonablycan be attrib- accumulated radiogenic gasesis given approximately by
uted to incorporatedair.
Aswill be shownlater,the ratioof (He/A)raa
R '• [(U/K) X 10a(3.0-{- 0.72(Th/U))] (3)
in all samplesinvestigatedin this report had
values of 1.6-130. Such a range in this ratio is which is independent of the time. It can be
believedto be characteristicof all natural gases, seen that R is not very sensitiveto changesin
and therefore,if a gasshowsa (He/A) ratio of
the Th/U ratio as a shift of from 0 to 4 in this
much lessthan this, it may be assumed,in lieu
of an argon isotopicanalysis,that the argon is
mainly nonradiogenicand has a corresponding
amount of atmosphericnitrogenassociated
with it.
quantity only changesR by a factor of 2. With
the exceptionof severalt;horium-richminerals,
I-IELIUM-ARGON
RA•IO
the Th/U ratio is rarelygreaterthan 10. Using
valuesof 3.5 ppm, 10 ppm, and 2.6 per cent for
the average rock concentration of uranium,
thorium, and potassium,respectively,we obtain
R---• 7. This is in rough agreement with the
If essentiallyall of the helium containedin
natural gasesis producedby the radioactive averageof the obscrvcd
results(seeTabl%l).
286
ZARTMAN, WASSERBURG, AND REYNOLDS
ixiO
•ø
8
6
I•109
2
8
I
3
6
4
•yrs
8
IxlO
8
ixlO7
0
I
I
I
I
I
I
1
I
2
3
4
5
6
7
R: (He/A•)
rod
Fig. 2. Variation of R -- (He/A),,a with time. (1) The ratio R of the productionrates at a
time r yearsago.(2) The ratioR of the total gasesproduced
in the intervalfroma time 4.5 X 109
yearsagountil r yearsago.(3) The ratioR of the total gases
produced
in the intervalfromr years
ago to the present.
concentrationsin an average igneous rock as
over time intervals longer than a few hundred estimated from selected values in the literature.
Inspectionof Figure 2 showsthat the time
million years, we have calculatedthis quantity
dependencyof R is rather slight. This ratio
for the followinginstances'
changesfrom 2.0 to 6.8 in the extreme cases
1. The ratio R of the productionratesat a time
considered.Except for the dependencyon the
In order to consider the effects of time on R
r years ago.
2. The ratio R of the total gasesproducedin
the interval from a time 4.5 X 10' years ago
until r years ago.
3. The ratio R of the total gasesproducedin
the interval from r years ago to the present.
Th/U ratio, similar curvesof this shapeare
generatedby any U/K ratio. They are simply
shifted along the abseissain proportionto the
U/K ratio. As indicatedabove,the rangein R
is a factor of 3.4. If the helium and argon con-
rained in traps in the upper sedimentarycrust
These curvesare shownin Figure 2. The pre- are composedof locally derived radiogenic
viously stated valuesof uranium,thorium, and material,we wouldexpectthe (HO/A4ø),•aratio
potassiumwere used to representthe present to be close to the instantaneous value. If these
HELIUM,
ARGON, AND CARBON
gasesare composedchiefly of material in the
processof upward transport from the deeper
crust and mantle, we might well be dealingwith
a (He4/A4ø)r•dratio somewherebetween the
instantaneous
value
and the cumulative
ones.
The validity of employingsuch an 'average'
rock as used above is indeed questionable,and
TABLE 2. Carbon isotopic composition of the
total gas, CH4, and C02 containedin somenatural
gases.Apparent temperaturescalculatedfrom Craig
[1953] for the pair CH4-C02 are also given. All
carbonisotopicdata are given as per mil difference
in the Cx•/Cx• ratio between the sample and the
Chicagostandard, PDB.
Calculated
NO.
i
•' Total
3
--36.0
-ss.s
--35.1
7
--40.2
•CH,
•CO,
T,øC
10
--39.7
--43.5
+8.1
11
...
--40.3
--•(•'9
12
13
14
...
...
. ..
--44.1
--46.4
--41.6
--7.9
--21.9
--31.9
--•'7
15
...
16
17
......
...
...
--•16
19
...
--44.1
20
...
-•o.s
18
21
25
26
27
28
......
--49.8
......
...
--•'2
--47.0
--i•14
......
--:•):9
--42.0
--38.8
--38.5
--39.6
--39.2
--40.0
.........
.........
......
36
......
--:•:2
--:•):5
-as.0
-4o.
38
--39.4
--41.5
39
--37.5
--38.5
40
...
--45.4
...
......
......
--41.7
......
--,•:0
5 powersof 10 in extreme rock types. If some
natural processesoperate over a sufficiently
large scale,they will tend to averageover such
variations. For example, carbonate rocks are
frequentlyintercalatedwith shalestendingwithin
a givenstratigraphicunit to cancelthe differences
obtainingin the pure lithologies'a 50-50 mixture
of a pure limestoneand a shale will yield a
rather normalU/K ratio. In general,it will be
...
...
-i•:4
-a9.s
30
31
32
33
34
......
139
242
-s.s
.
29
35
--3.9
--4.1
71
--41.0
.a
ratio one hundred times less, and, in addition,
exhibita rather constantU/K ratio.
The possible
rangein the U/K ratio is about
......
......
......
--40.1
On the otherhand,an evaporiteproduced
from
uncontaminated
sea water would yield a U/K
of carbonates,however, the normal rock types
......
--37.2
it is necessaryto considerin more detail the
abundancesof the radioactive elements in
differentrocks.A compilationof the abundances
in various lithologies is presented in Table 3.
Wide variationsin concentrations
and U/K and
Th/U ratios are apparent. For unusual rock
types (Kolm from Sweden), or where the rock
is the host for a mineral deposit (Colorado
Plateau uranium deposits),the ratios may vary
over many powers of 10. Of the normal rock
types tabulated, it is seenthat the purer carbonates, although not rich in uranium, have very
low potassiumcontentsyielding ratios as much
as one hundred times greater than granites.
may have a Th/U ratio closeto zero.Exclusive
.........
.........
s
9
287
...
...
...
--5.9
--15.0
+i•'8
',i•
--8.8
185
exceedinglydifficult to relate the radiogenic
helium and argon in any reservoir to a particular rocksource,sincethesegasesundoubtedly
will have migrated over some distances and
throughdifferentrocktypes.In certain instances
it may be possibleto showthat thesegasescould
havebeenproducedwithin their presentreservoir,
but this will not be a uniqueattribute. Considering the wide variations possiblein R, owing to
variationsof the U/K ratio, it is remarkable
that
the
measured
values
lie
within
such
a
restricted range. It is our interpretation that
this is becauseof the large- and small-scale
averaging inherent in the processesof accumulation with the total sourcehaving a 'normal'
U/K ratio.
One of the important factors concerningthe
He and A to be investigatedis the escapeof
these gasesfrom the minerals in which they
originated. That A•0-K •0 and helium age-dating
methods work at all attests to the retention
of
at least someof thesegasesin the sourceminerals.
288
ZARTMAN, WASSERBURG, AND REYNOLDS
ItELIUM,
ARGON, AND CARBON
Much work has been done by Keevil [1941]and
TABLE 5.
Hurley and Goodman[1941]on the lossof He
289
Henry's Law Constants for
Several Selected Gases
from individual minerals and total rock, and
corresponding
workby Wasserburg,
Hayden,and
K, = •/•'
Solvent
Jensen [1956] and Goldich, Baadsgaard,Nier,
and Hoffman[1957]yields someinformationon
the argon retentionability of severalcommon
Fresh Water
potassiumminerals.Somedata on I-Ie and A
Gas
lossesfrom mineralsand rocks are prcsentedin
Table 4. Since,in general,the observedhelium
N 2
and argonlossesare roughlyproportionalto the
helium and argon contents,respectively,of the
mineral,the (He4/A4ø),aa
ratio of the 'available'
A
Hc
CH 4
15øC
50øC
Marine
15øC
56
87
25
113
40
Ill
33
......
73
27
46
......
Water
50øC
114
53
gaswill be equalto the (He4/A4ø)production
ratio in the total rock multiplied by someconstant,whichwill betakento be time independent. has retained80 per cent of its argon and 40 per
cent of its helium is later completely outgassed
The value of this constant will be the fractional
by
metamorphismor fusion, the gas thereby
helium loss,•H., divided by the fractionalargon
evolved might be preferentially enriched in
loss•A. As an approximationwe might expecta
freshigneous
rockto lose3/5 its Ite4 and 1/5 argon by a factor of 2. Thus, from an average
its A4øinto interstitial pore spaces.Weathering igneousrock we might expect extreme 'available'
ratiosof about3 and 20, depending
and aliagenetic
processes
can do little to increase (I-Ie•/A•ø)r=a
on whether the gasesrepresentedthe value atthe alreadyhighyield for Ite4from the rock,but
they offerampleopportunityfor breakingdown tained by compleh•outgassingof a rock which
potassium-bearing
mineralsand releasingmuch had formerlypreferentiallylost helium or by the
retainedargon.Thus,we mightexpectsediments low temperature diffusionof helium and argon
to favor the outgassingof argon more highly out of a young rock. If the sourcerock differs
than an equivalentfreshigneousrock. The rela- considerablyin its K, U, and Th abundances
tive rates of escapeof helium and argon from from an average igneous rock, the resultant
ratiowill, of course,reflect
rocks at elevated temperatures and pressures available(I-Iea/A•ø)r=a
this. Gases originating in pure carbonate rocks
are poorly known. However,a more complete
loss of both gasesmight be expectedunder may have high values of R.
A number of gas transport mechanismscould
conditionsof metamorphism.If a rock which
TABLE 4. Helium and Argon Retentivity in
Some Common Rocks and Minerals*
water,freshwater,or petroleum,the (He4/A40),,a
Fractional Retentivity
Mineral or Rock
Helium
Quartz
Feldspar
0.33
0.25
Mica
0.50
Pyroxene
Magnetite
0.75
1.00
Hornblende
"Granite
Diabase
1.00
0.40
0.60
producea fractionation of argon and helium. If,
during the time of gas accumulation,the helium
and argon are transportedby solutionin conhate
Argon
0.75
1.00
ratio that we actually observein a natural gas
samplewill be influencedby the relative solubilities of thesegasesin the transportingmedium.
If diffusive equilibrium is attained, we would
expect the followingrelationshipto hold: •
R'-- Ku.
Ro= K,R
o
KA
(4)
where R ø and R• are the atomic ratios of the
helium to argon in solution and in the gas
reservoirrespectively,and Kn. and K,• are the
Henry's law constanHfor helium and argon,
* Referencesincluded Keevil, 1941; Hurley and
respectively.The Henry's law constants for
Goodman,1941;Wasserburg,
Hayden, and Jensen,
1956; and Goldich,Baadsgaard,Nier, and Hoff- severalgasesat various•emperaturesare given
in Table 5. Rakestrawand Erareel[1938]showed
man, 1957.
290
ZARTMAN,
WASSERBURG, AND REYNOLDS
that argonis only about80 per centas solublein of certain potassium evapori•e salts such as
marine water of normal salinity as it is in fresh sylvite (K --• 50 per cent) and somevery pure
water.Similarwork by z[kerlof[1935]on the carbonaterocks (K <( 0.01 per cent), the potaseffect of salinity on helium solubility suggests sium content of most rocks generally showsless
about the same behavior for this gas. The than an order of magnitudevariation. This maior
solubilityof argonand heliumin crudepetroleum rock-forming element makes up from 1 to 4
is unknown, as is the importanceof noble gas per cent of many sedimentaryand igneousrocks.
tr•ansportby this means.in water solutionsfor However, uranium and thorium, which generally
temperaturesof 50-80øC, K • ___•2.8, and, there- occur as trace elements, are subject to wide
fore, the gas phasewould be enrichedin helium variations in abundance,and the possibilityof
relative to argon as comparedwith the solution. local enrichmentsmust be considered.Indeed,
Thus, if much more gas is containedin solution one of the early theoriesproposedfor the occurthan occursin the gas phase, the equilibrium rence of high helium wells attributed the high
ratio of He/A in the gasreservoirwouldbe 2.8 helium contentto an underlyinguranium deposit
times greater than the 'available' ratio. A more [Rogers,1921].If an order of magnitudeor larger
rigoroustreatment of this problemis given later enrichmentin the helium contentof a natural gas
is affectedby this process,the gas shouldshow
in the paper.
The differencesin solubilityof variousnatural an abnormallyhigh (He/A)r•d ratio. The large
gas componentscould in principle, through a increase in the abundance of a major rockmultistageprocessof solutionand effervescence, forming element suchas potassiumas would be
cause considerable variations
in R. It would be
possibleunder ideal conditions to produce a
range in R of over 20 by employingonly three
stagesof distillation.It is not possibleat present,
however, to say how effective multistage distillation actually is in achieving variations in R.
That many stagesof distillation do not operate
is indicatedby the limited range observedin R.
If compositionalvariations are ignored, it is
seenthat the effectsof the other parameterswill
permit over an orderof magnitudevariationin R.
Thus, the value of R actually observedin natural
gas may range between extremes of at least
3-50 without any need for assuming compositionaldifferences.
All of the samplesanalyzed
have values of R that fall within the range of
1.6 to 130. Most of thes8resultsagreequi•e well
with this model, assumingonly minor variations
in the U/K ratio. It is, of course,possibleto
attribute
the observed variations
in R to com-
needed to maintain the observed(He/A)r•d
ratios
could not occur in common
rocks. That
such an enrichment in radiogenic helium over
radiogenicargondoesnot exist in the caseof the
Texas Panhandlegasfield was shownby Wasserburg, Czamanske, Faul, and Hayden [1957].
The resultsof the presentinvesfigafio•also
tend to disprovethe idea of high enrichmentsin
helium
due to abnormal
uranium
or thorium
concentrationsfor a number of high helium gas
fields.
The problemof recognizingthe contributionof
gasesfrom the mantle or lower crust is extremely
difficult. This is particularly true for helium
and argon, since the earth is a highly differentiated body and the productionof theseelements
is dependenton the U, Th, and K concentrations.
The ratio R in modern material
of chrondritic
compositionis about 1; the ratio R in an average
igneousrock 4.5 billion years ago would be 2.0
as compared with the present value of 6.8.
positional differencesinstead of diffusion and
solubility effects. It is not possibleat present
Someof the lowestvaluesof (He•/A•o)r•were
to say preciselywhich factors account for the in CO• wells that are associatedwith igneous
variation.
activity. These values are in the direction
Damon and Kulp [1958] have measuredthe effected by great age or chondrific production
ratio of excessradiogenichelium to argon in rates. These results, of course, are only sugberyls and cordierires. They obtained ratios gestiveinasmuchas they couldbe producedby
ranging from 0.5 to 130 with an averagevalue a variety of mechanisms.
It should be noted that the argon from the
of 20. Our resultson well gasesare thus rather
similar to the values fpund for these trapped CO• wells is extremelyradiogenic.If this represents juvenile argon, it indicatesthat very little
magmatic gases.
It mustbe pointedout that, with the exception A • is associatedwith it. This is compatiblewith
HELIUM,
ARGON, AND CARBON
the resultsderived from the solubility model for
air argon. The latter data indicate that no
significant amount of atmospherictype argon
occurs in these gases above the amount that
would be present from original equilibration
with air. We infer from this that no deep-seated
gases are contributing significant amounts of
argon of this composition.
NOBLE GAS ABUNDANCES IN NATURAL
GASES
The helium content of the gas samplesinvestigated varies between 37 and 62,200 ppm
and the radiogenicargon contentvariesbetween
3.7 and 5580 ppm. Sinceany attempt to explain
the occurrenceof the rare gasesin natural gases
must account for the absolute amounts and
concentrations as well as for the ratio of radio-
genichelium to radiogenicargon,factorsinfluencing helium and argon abundances will now be
discussed.
Whereasthe (He/A)r• ratio is only weakly
time-dependent over times comparable to the
age of the earth, the actual productionof these
gasesis stronglytime-dependent.The radiogenic
helium and argon content of a natural gas
reservoir is not necessarilyproportional to the
age of the source rock, but is, rather, a complicated function of the accumulationhistory
of the gas. It is possiblethat much of the radiogenic gases are incorporated into the natural
gas by a sweeping-upeffect during the time of
migration from the sourceto the reservoir rock.
In such an event, the He and A content of the
rocks at the time that they were traversed by
the accumulatinggaseswould be an important
factor. If there was little noble gas escape
beforegas migration, the length of time between
rock formation and petroleum accumulation
would determine
the concentration
of He and A
in the pore space.Studiesof a number of oil and
gas fields have shown that this time between
sourcerock depositionand petroleum migration
may vary from between tens of millions and
hundreds of millions of years. The outgassing
of very old basement rocks through metamorphism would be a possiblesourceof high
concentrationsof these radiogenic gases. Such
gasesmay leak into a sedimentarysectionthrough
various fractures or faults. It may also be true
for somecasesthat the noblegaseshave escaped
continuouslyfrom the rocks during all times
except the period of gas accumulation.In this
291
event, the important time factor would be the
length of the time interval over which gas
migrationtook place.
Once the gaseshave been removed from the
crystal lattices they become available for
migration either by solutionor by gaseoustransfer. Under equilibrium conditions,the distribution of any gas between a gaseousand liquid
phaseis related approximatelyby Henry's law.
It is, of course, questionablethat equilibrium
conditionsprevail over large distancesbetween
the accumulated gases and interstitial pore
fluids.It will, however,be assumedin the following discussionthat diffusiveequilibrium obtains
for the rare gasesand the consequences
of such
a model will be investigated.
Equilibrium model. Goryunov and Kozlov
[1940] have pointed out the importance of
solubility phenomenain natural gas accumulation, and part of the followingdiscussion
parallels
their work.
Let us consideran equilibrium reservoirmodel
in which the rock has a porosityp, and suppose
the pore spaceto be occupiedby both an aqueous
phase(s) and a gasphase(g), all under a hydrostatic pressureof P atmospheres.Let the volumes
occupied by the gas and aqueous phase be
V, and V', respectively,and the concentrations
of gasspecies
i in eachof thesephasesbe C•, and
C•,, in units of standard cc per cm8. The total
amount of speciesi in the pore system is then
CdV' • C•'V •, and the fraction fd of this
speciesin the gas phaseis
C•• V •
= c, v + c,'v'
(5)
Supposethat each speciessatisfiesa Henry's
law relationshipof the form C•, - K•C• • where
K• is the Henry's law constant for speciesi.
We then have
=1/(1-]-K,V'••)
(6)
Since KH.---• 110 and KA ----- 40 for aqueous
solutionsat 50øC,fH.• and fA• will be closeto 1
when more than about 5 per cent of the pore
spaceis occupiedby a gaseousphase.
In the units used, Cd is (assumingideality)
numerically equal to the partial pressurein
292
ZARTMAN,
WASSERBURG,
atmospheresof speciesi. The condition that a
pure gas phase i exists when the hydrostatic
pressure
is P is P,/K• -- C••. It followsthat for
any given concentrationC•• there is a maximum
depth at which a gas phaseof pure i may exist.
If we consider an infinitesimal
helium bubble at
AND REYNOLDS
to be due to the releasefrom connateor ground
waters which were originally saturated with
argon under atmospheric conditions. If the
system is closedsubsequentto burial, we have
C•'øV'ø = C.'V' + C.•V •
1000-footdepth under a hydrostatic pressureof
30 atmospheres pressure, this will require
Cue• --__0.3 cc STP/cc pore vol. If we assume
the porosity to be as low as 10% this will correspondto 3 )< 10-• cc STP of He per cc of rock.
As will be shown later, such a value is obtainable
only under extreme conditions.It is, of course,
obvious that if, in a certain environment,
radiogenichelium had associatedwith it another
componentnot subsequentlyremoved,we would
not find a pure helium gas.
If we let C• equal the •nean concentrationof
gasspeciesi in the total porespaceof the system,
then, from (6) we have
c, =
-
-
(7)
From (7) we see that if all of the gas speciesi
occursin the gas phase the mean concentration
will simply equalthe concentrationin the gaseous
phase. If, however, because of the rather insoluble nature of some gases,we have most of
the gas dissolvedin an aqueousphasewith only
a very small gas bubble in equilibrium with it
(f?----- 0), we gain a factor of K• in C•gover the
mean pore space concentration. Thus, an
infinitesimal bubble of helium in equilibrium
with a liquid at 50øCwill have a He concentration
of 110 times higher than the new concentration.
As will be shown below, most reservoir gases
appear to have significantamountsof gas in a
dissolvedphase.
The previous model has treated only a gas
with an aqueousphase.For gasesassociatedwith
a liquid petroleum phase,the equilibrium relationship involving this phase in addition to an
aqueousand a gaseousphasemust be considered.
This will obviously increase the number of
variables in the model caluclations. The effect of
water salinity must also be taken into account
in a more precisecalculation.
In addition to the radiogenicnoble gas content
of natural gases,argonhaving the compositionof
present-dayatmosphericargon is found to be
present in all of the gas samples.As discussed
previously, the origin of this air argon is uncertain. For purposesof discussion,we assumeit
(s)
wherethe subscripta representsair argon.C,•,ois
the initial concentrationof air argon in solution
and V ,0 is the initial volume of the solution.
If we assume that the volumes of the initial
and final water bodies are equal, we obtain
-'7
V = K.•C,•
•ø-- C,•"
(9)
In a diffusiveequilibrium model, the systemis
assignedvaluesof V ' and Vg. This is, of course,
unrealistic, since the natural system will not
have sharply defined boundaries. The ratio
V•/V • ascalculated
will therefore
applyfor some
effective volume over which equilibrium is
attained. Under atmosphericconditionsat 15øC
and in equilibrium with ocean water of normal
salinity,we have C,,'ø• 3.0 X 10-• cc STP/cc
H•.O [Rankama and Sahama, 1950]. The value
of C• is determinedby the measuredconcentration of air argon in the natural gas at the well
pressure.
Substitutingthis expression
for V,/V •
in (6), and using K• = 53 (for 50øC and normal
marine salinity) we have for radiogenicargon
IAg
1
$
K•. V"
= 1-- K•.C,,,o
•___
1 -- 630•
• (10)
and for other arbitrary species
1
I, • =
•
C• K,•
(11)
l + K•(K•.C•O_
C•)
We see that for the assumed equilibrium
model it is possibleto calculate the fraction of
speciesi which is in the gas phase.This may
be applied to other gasesas well as to radiogenic
He and A. In Table i are given the calculated
valuesof V'/V ,• and f•
for the fractionof
radiogenicargon occurringin the gaseousphase.
Because of the many obvious uncertainties
ttELIUM,
ARGON, AND CARBON
293
All the parametersgiven in (14) are expressed
as the effective values for the system under
investigation. Substitution of reasonable estimates for these parameters yields radiogenic
heliumand argonconcentrations
in natural gases
quite consistentwith actually observedvalues.
of fAo and V'/V• appearto be quitereasonable. For example, let us calculate the radiogenic
Only two samples(11 and 24) give impossible helium and argon content of two extreme reservalues of fA•, and these discrepanciesare not voir cases.Let us assumea rock density of 2.5
extreme.
and an effective U, Th, and K concentrationin
If there were originallyentrappedair bubbles the rocks of 3.5 ppm, 10 ppm, and 2.6 per cent,
in the pore space,or if V' < V 'ø was due to respectively. In the one instance, let us set
hydrarich reactions in diagenesis,then the v -- 5 X l0 s yr., •jUo= •j, --• 1, fUog•-' fig•-•- 0,
calculatedvalueof Vø/V•, in (9) will be larger P = 10 atmospheres(147 psia), and p = I per
than the true value. The calculated values of
cent. This correspondsto an environment in
f• will be too small, owing to such effects. which the total radiogenic noble gas content
Next, let us look at the effect of porosity, p, producedby one-half billion years of decay is
on concentration. If we assume the mean noble
completelyreleasedinto rock of 1 per cent mean
gas concentrationin the pore space,C•, to be porosity having a hydrostatic pressureof 10
esserrtiatt
'
tmospheres:GeologicM!•this couldbe brought
centration producedin the mineral phaseper cc about by the continuousrelease of radiogenic
gases into overlying sediments of one-half
of rock, we have
billion-yearage, or by the completeretentionof
such gases in basement rocks over this time
C,= •-•
N,
(12)
P
interval, followed by some metamorphic event
whereC•, as before,hasthe unitsof cc STP/cc which then releasedthe gas. This former possiof porespace,N• hasthe unitsof cc STP/ccof bility would appear unlikely where no sedimensystem, and • is the rock degassingfactor. tary cover of sufficientage was presentto trap
The concentration of the noble gases in the the continuouslyreleasedgas.The gasis virtually
porespaceis inverselyproportionalto the poros- all dissolvedin an aqueousphase,and thus the
ity. N• for radiogenicheliumand argonis given concentrationin the gas phaseis increasedover
by (2a) and (2b), respectively,calculatedper cc the mean pore space concentration by the
of rock. The molecular abundancein ppm of numerical value of the Henry's law constant.
These conditions yield Fuø •--- 106 ppm and
speciesi in the gasphaseis I'•,
F•___ 5 X 104ppm.
For contrast, let us considera case in which
r = 5 X 107years,•Ue•'--•,•-•- 0.5, fuoø•f•.o •-•- 1,
where P is the pressureof the reservoir in P = 100 atmospheres(1470 psia), and p = 5
atmospheres.This equation assumesthe gases per cent. This correspondsto a situation in
to be perfect. For hydrostatic pressureP •--- which one-half of the radiogenicnoble gas pro0.030h,whereh is reservoirdepth in feet below duction over a 50-million-yearperiodis released
into rock of 5 per cent mean porosityhaving a
the surface.
Combining (2), (7)' (12), and (13), we have hydrostatic pressureof 100 atmospheres.These
conditionsyield I•Ho-- 10 ppm and Fx = 1 ppm.
•H.(0.120 U --I--0.029 Th)r
If, instead,the gasweremainlydissolved(f--, 0),
regardingthe assumptions
involved,it is doubtful
that the calculationsare strictly applicableto
real natural gas reservoirs.Nonetheless,such
considerations
are usefulas a meansof comparing
the behavior of natural systemsto the simple
idealized model. Usually the calculatedvalues
F,- p X 106
rile
P.p
ß(KuoFA --
(13)
[Ka•-
1]]a•)
P.p
[K.•-
An average reservoir might have effective
valuesof •' = 10• years,•Uo= 4/5, • = 1/2,
•^(3.99 X 10-6K)r
ß(K.•-
(14a)
the value of FHo would become a factor of 100
larger.
1]]•)
fH,o • 0.9, f,•o •___0.75, P --' 50 atmospheres
(735 psia), and p = 3 per cent. Assumingthe
and rock density
(14b) sameU, Th, and K abundances
294
ZARTMAN, WASSERBURG, AND REYNOLDS
as above, we have FHo= 1040 ppm and FA ---86 ppm with R---• 12. In this case we have
80 per cent and 50 per cent of the accumulated
radiogenic helium and argon, respectively,
resulting from one hundred million years of
decay being releasedinto rock of 3 per cent
mean porosity. The values of f? correspondto
of the reservoirrock is 2 per cent, the effective
porosity of the entire sourcerock system is
muchlower. This might be expectedif most of
the helium were derived from a very tight
basementcomplex.It is difficultto tell which
are governingfactors in this case. At present,
we surmise that this gas has probably been
havingV./Vo_____.
10.
produced
by the favorable
conjunction
of several
of these effects. It should be emphasizedthat
of rock swept out during the formation of a it is rather difficult to account for this factor
g•s reservoir,let usconsiderthe helium-producing of 10; if an additionalfactorof 10 wererequired,
zone of the Rattlesnake Gas Field, San Juan it wouldbe impossible
to obtainthis reasonably
County, New Mexico. W. M. Deaton, chief in terms of the presentmodel.
helium consultant for the Bureau of Mines
If we comparethe San Juan high helium gas
Helium Activity (personalcommunication)esti- with that from the Texas Panhandle, we see
mates the total original volume of the reservoir that the formergasnot only containsmore than
gas to be about 2.4 X 109cubicfeet at 15.0 psia a fivefoldhigherabundancein helium, but also
due to
and 60øF. About half of the total gas has been a 35 times greaterheliumconcentration,
removed,and the field is not being producedat the higher pressureof the San Juan gas.
It shouldbe pointedout that a gasthat existed
the presenttime. The gas, which has a helium
contentof 7.6 per cent, was producedfrom the at rather shallow depths with a particular
Leadville-Ouray(Mississippian-Devonian)
form- abundanceof helium and subsequentlytransations and had an initial pressure of about portedas a' closed
systemto a greatdepthwill
3000 psia. Thus the reservoiris approximately retain the samehelium abundance,but it will
Such
1.2 X 107cubicfeet, and assumingp: 2 per cent have a muchhigherheliumconcentration.
In order to make an estimate
of the volume
and V'//V g• 94 (approximate
valuesof near-by a gaswill, of course,not be in equilibriumwith
Navajo C-1), we see that this corresponds
to a the surroundingaqueousreservoirand will
volume of V'/p of 5.6 X 10•ø cubic feet, or ultimatelyreturnto a lowerheliumconcentration.
0.4 cubic mile. Assuming an average uranium
and thorium abundance in the rock •Ho --- 1,
and an effectivev of 3 )• 10s years, we calculate
from (14) that Puo---• 7000 ppm. This is lower
than the observedvalue by a factor of 10.
Let us now lookat somepossibleexplanationsfor
There is no evidencesupportingsuch a trans-
portationhistoryfor the SanJuangas.
We have seen that under equilibrium conditions the concentrationof a slightly soluble
gassuchas He or A in the gaseous
phaseis of
the order of 10stimes its concentrationin solution.
this discrepancy.
Sincethe gashasa (He/A)r•d The uniform release of these gases from a
ratio which is characteristic of common rocks, homogeneous
sourcerock into a systemwhich
we cannot reasonablyassumethat either the containsboth poreliquid and gaswould,in the
uranium or thorium concentrations, or both, absence
of completediffusiveequilibrium,tend
in this sourceare abnormally high, although a
factor of 2 increasemay well occur. Although
the gasreservoiris in Pennsylvanianstrata, it is
possiblethat the radiogenicfraction of the gas
was chiefly derived by the outgassingof much
older basement rock containing a correspondingly higher helium concentration.Also, the
possibilitythat our equilibriumsolubilitymodel
to make C•g < K•C•'. Under such conditions,
the concentration
of speciesi in the gas phase
would be lower than expectedby diffusive
equilibrium.Except for minor effectsbrought
about by the temperaturedependency
of the
Henry'slaw constant,it is difficultto envision
a natural situation which would tend to make
C•,' >
K•C• o. With referenceto radiogenic
helium,only if a sourcematerialis selectively
in the helium concentrationin the gas phase contributingthe helium to a gas phase at a
is incorrect would allow for a twofold increase
above that calculated. A favorable combination
rate of KHo times faster than it is feeding a
of these three effects could just allow for the
observedhelium concentrationin the gas. In
liquidphasecanC•ø•be greaterthan
Radon. The existence of radon in many
beenusedto infer
addition,it is possiblethat whereasthe porosity naturalgaseshassometimes
HELIUM,
ARGON, AND CARBON
the presenceof a concentration
of uraniumin
the neighborhood
of suchoccurrences.
Faul, Gott,
Manger, Mytton, and Sakakura[1954]have reportedon the Rn contentof somehelium-rich
naturalgases.Theseworkersconclude
that it is
uncertain whether the high helium content of
certain gasesis related to the presenceof Rn.
This problemis of interestin considering
the
possibilitythat a significantportionof the He
in somenatural gasescan be the productof U,
Th decayin their presentreservoir.
Sakakura, Lindberg,and Faul [1959] have
reportedthe radon contentin the gasesfrom
295
will considergasesas incompressible,and take
the rate of productionQ in units of cc volumeat
well pressureper unit time.
For the caseof the uniform productionof Rn
within a cylinderof radius R centerabout the
origin and no productionoutsideof this region,
the value of C at the well is
C(0)= q/X1- exp Q I
(16)
It is clear that C(0) is insensitiveto changesin
the parametersat distancesmuch greater than
r--
(- Q/X•-Hp)i, i.e., distancesfor which
the time of travel to the well is equal to the
production,star•ingfrom a situationwherethe mean life. This distance is probably not in
four wells as a function
of the cumulative
wells had been shut down for 2 or 3 weeks.
The net gas flow requiredbefore the Rn concentration reached a constant value was small
and of such a value as to indicate that the
principalsourceof the activity was not in the
well hole itselfbut immediatelyadjacent.These
workershave presenteda theoreticaltreatment
of the problemof the gastransportof Rn. They
excess of a few hundred
feet for most wells.
The highestRn concentrationin the Panhandle
helium wells reported by Faul, Gott, Manger,
Mytton, and Sakakura [1954]is 500 micromicrocuriesper liter at STP. Usinga well headpressure
of 16 atmosphereswe obtain for a uniform
sourceq = XC-----0.24Rn decay/sec.cc of pore
space.Assuminga porosity of 10 per cent, this
conclude that for the Texas Panhandle gases correspondsto an emanating uranium con-
the Rn is due to a uranium concentration of
centrationof 10-8 g U/g rock.If but I per cent
between0.4 and9.0 ppm,assuming
an emanating
powerof 10 per cent.
In the following,we will presenta simplified
treatment of the steadystate transportproblem
that will sufficefor the purposeat hand.
The equation governing the steady state
concentrationof Rn in a fluid phase for the
caseof cylindricalsymmetryis givenby
of the emanation escapesto the pores, this
corresponds
to a uranium concentrationof only
100 ppm. For a well in the San Juan basin
containing6 per cent He, Faul, Gott, Manger,
Mytton,andSakakura[1954]reporta Rn activity
of only 5 micromicrocuries/liters
STP.
In some of the casesreported, the Rn level
was sufficientto accountfor the helium present,
assumingsteady productionfor 10s years. In
others, it could contribute only 0.1 per cent
=
--kpC(r)
-•pq(r)
(15)
r
dr
of the He. Since more He may escapefrom the
Here C(r) is the concentrationof Rn in the reservoir rocks than is indicated by the Rn
fluid phaseat distancer, J(r) is the outward concentration,it is quite possible that some
radial volume flux of fluid with the dimensions
gasesobtaintheirheliumaftertheirfinale_ntrapof velocity,p is the porosity,q(r) is the rate of ment. In any case,the Rn data do not support
generationof Rn per cc of pore space,and the case for the generation of these helium
•, is the decayconstantfor Rn. If the fluid is containing gasesfrom high uranium concentra-
incompressible,
J(r) = Q/2•-Hr, with -Q/H
tions.
Helium distributionfunction. There are about
being the volumeof fluid yieldedby the well
3400heliumanalysesfor natural gasesasreported
per unit heightof the producing
horizon.
If the productionrate of Rn is everywhere by the U.S. Bureau of Mines [Andersonand
constant,we seethat C = q/•. The Rn activity Hinson, 1951; Boone,1958]. The gasesanalyzed
is then constant and equal to the production comefrom a wide variety of geologicalsituations
rate. If the fluid is compressible,
the concentra- and, excluding the very high helium wells,
tion is a more complicatedfunctionof position shouldbe a representativesampleof natural gas
due to the fact that at lower pressures
the fluid accumulations.Becauseof the large number of
occupiesa greatervolume.For simplicity,we analyses,it shouldbe possibleto get a rather
296
ZARTMAN,
WASSERBURG, AND REYNOLDS
i•iO-2
6
IxlO-3
0.01
I
01
, ,I
i
I
5
I
I0
I
20
I I I ! I
40
60
I
80
1
90
I
95
I
99
I
99.9
99.99
A(x)
Fig. 3. Log probabilityplot of the helium abundancein natural g•ses.Statisticsfor abundanceslessthan 0.8 per centand estimatesfor high heliumproductionare taken from U.S. Bureau
of Mines data.
productionhas an averageHe concentrationof
1.5 per cent [U.S. Bureau of Mines, 1959].
Using these data, a frequencyhistogramwas
constructed.The cumulative data, plotted on
log probability paper, are shown in Figure 3.
It is evident that a lognormal distribution
representsthe data fairly well. The percentties
productionand theirheliumcontent,weobtained were determined [Aitchisonand Brown, 1957].
the result that 3 per cent of the total gases usingthe straightline drawn throughthe points.
contained helium in a concentration of over
The following values were determinedfor this
good descriptionof their frequencycurve. The
data used were 3000 analysesof samplesconraining less than 0.8 per cent He. Becausean
excessivenumber of samples were analyzed
from known He-producing areas, we have
discardedall analysesreportingover 0.8 per cent
He. Using estimates of the total annual gas
0.8 per cent. This fraction of the annual gas
curve:
HELIUM,
ARGON, AND CARBON
29?
gases is among the lowest observed in any
analyzedsamples.The atmosphericargoncontent
median ----0.0610•o
•r -- 1.59
of thesegasesrangesbetween44 and 124 ppm;
the nitrogenvaries from lessthan I per cent to
mode- 0.0049•o
/• - 2.80
over 30 per cent. Geologicevidenceindicates
Figure 4 illustrates the frequency curve and that the reservoirrocksmay be in communication
usestheseparametersfor the frequencyfunction with ground water circulation. These Tertiary
and Cretaceous sediments crop out a short
distance to the east along the western foothills
of the Sierra Nevada. The young age of the
sedimentsmay account for the low radiogenic
This curvemay be comparedwith the histogram. noblegascontentof thesenatural gases,and the
The histogram does not appear to have possibilityof open systemconditionsor heterobimodal characteristics. This observation further
geneity in the sourcematerial is suggestedby
suggeststhat the high He gasesdo not (in a the large variationsin the (He/A),• and
statisticalsense)representany specialmechanism, (N•/A, ir) ratios. Chemically,the Sacramento
but rather representlow-probabilityevents on Valley gasesare extremelydry, with methane
the tail of a continuous-probability
curve.
making up essentially all the hydrocarbon
mean -- 0.2170%
dA(x)
I exp--[[(ln
x-dx -- 2•ro'x
.•/•
(17)
fraction and nitrogen varying between 1 and
2n no.oo•f. mh..+...11f.hi.• nitrogen cannotbe nr
direct atmospheric origin has been discussed
SAMPLES
Samples1-7 are gasesfrom Eoceneand Cretaceoussandsof the SacramentoValley, California. previously.Sample6 from the Marysville-Butte
They have a wide range in (He/A),• and field was taken from gas closelyassociatedwith
(N•/A•i,) ratios, and their e is rather low. The Pleistocene volcanic extrusives.
Samples8-10 and 36-39 are gasesoccurring
radiogenichelium and argon content of these
26
24
22
2O
0
- I
-2
-5
-4
log X
Fig. 4. Frequencycurve and histogramshowingthe distributionof helium in natural gases.
298
ZARTMAN,
WASSERBURG, AND REYNOLDS
in rocks of Cretaceousto Eocene age from
has selectivelyenrichedthe gasphasein helium.
northwestern
The low value of Vo/Vg may in reality point
Colorado
and
southwestern
Wyoming. Although thesegasesalso show wide
to the removalor absenceof air argonfrom the
variationsin (N•/A•ir) and e, their (He/A)r•a system. If the high value of R did result from
ratio remains fairly constant despite over an a distillationmechanismoperatingon an originorder of magnitude variation in radiogenic ally normal (He/A)•,a ratio, thereshouldexist
helium and argon content. Whereas the chief somewherea complimentary phase having a
factor producing the spread observed in the correspondingly
low valueof R. It is alsopossible
(N•/Asi•) ratio for the Sacramento
Valleygases that the sourcesof the western Appalachian
was a highly variable nitrogen content, the gasesare characterized
by a low K/U ratio.
Green River gases,which demonstratean even Whereassucha ratio might be expectedto occur
greater range in (N•./Asi,) ratios, have more in impure limestones,the fact that most of the
uniform nitrogen content, but over an order of Appalachiangasesoccur in stratigraphictraps,
magnitudevariation in air argon content.Thus, involving sandstonelensessurroundedby imthesegasesmay representeither (1) reservoirsof perviousshales,seemsto excludethis possibility.
highlyvariable¾ø/¾•,(2) reservoirs
int• which In no way does this region demonstrate a
air argon from sources other than saturated (He/A)•a ratio to be expectedby a simple
seawater wasintroduced,(3) reservoirsin which degassing
of a chondriticmantle.
varying degreesof diffusivedisequilibriumexist,
Samples 11 from the Sexsonfield, Kansas,
or (4) sampleshaving incorporatedair argon 22 from a wildcat well in San Juan County, New
owing to varying levels of contamination.
Mexico, 24 from the Keyes field in Oklahoma,
Samples12 and 13 were selectedt• represent and 29-32 from the West Panhandlefield, Texas,
gasesfrom a present-daygeosynclinalenviron- were selected as representative high helium
ment.
natural gases. They range between 0.5 and
Samples18-20 are from Lea County, New over 6 per cent helium with (He/A),,a ratios
Mexico. The first two gasesare producedfrom ranging from 11.1 to 23.3. Such valuesof R are
Permian sandstones.Sample 18 comes from a in no way compatiblewith a large increasein
reservoir directly overlying the reservoir of the concentration of uranium and thorium over
sample 19 and is stratigraphically separated potassiumas comparedwith averagerocks. As
from it by about 1000 feet of section.The other stated elsewhere,it is the authors'opinionthat
gasoccursin a Permiancarbonate.The similarity high helium wells result from the favorable
in the (He/A)•,a ratio, the (N•/A,i•) ratio, and interaction of several processesoperating on
e for thesesamplesfrom the sandstonereservoirs normal rock types having averageU, Th, and
may be a reflectionof a similar source.The gas K contents.Someof thesehigh heliumreservoirs,
from the limestx)nereservoir appears to be such as those of the Texas Panhandle field,
distinctly different in these parameters from appear to be closely related to buried Prethe other two gases.
cambrian granite and granite wash, which may
Sample23 fromwesternNew York andsamples have been the sourceof the helium [Cornerand
25-27 from Pennsylvania represent Paleozoic Crum, 1935]. In other areas,suchas San Juan
gasesfrom the Appalachianpetroleumprovince. County, New Mexico, no suchcorrelationwith
Thesegasescontainthe highest(He/A)•a ratios basement rock is obvious tHinson, 1947]. In
found in any of the samples;they range from many cases,however,the high heliumreservoirs
20.2 to 134. Sucha high ratio is quite surprising are stratigraphically low in areas containing
in these gasesbecauseof their relatively great severalreservoirhorizons.This wouldbe expected
age, low ¾ø/¾%and proximity t• orogenic if helium, having been liberated from the baseactivity, which may have been more effective ment, rose through the overlying sediments
than averagein releasingradiogenicargon. All until it became incorporated into the first
of thesefeaturespoint toward a low (He/A)r•d reservoirit encountered.Numerousexceptionsto
ratio.
the occurrenceof high helium gases in low
On the other hand, it is possiblethat some stratigraphichorizonsexist. Rogers[1921] found
distillation mechanismhas been particularly that in the mid-continentregionit may be that
operative in the productionof these gasesand certainintermediatehorizons,or eventhe highest
HELIUM, ARGON, AND CARBON
producingzone,containthe largestpercentageof
helium. It is still not possibleto give a rigid
explanation of the processesby which certain
reservoirsbecame highly enriched in helium.
These gasescontain between 50 and 200 ppm
of atmosphericargon. A comparisonbetween
299
Sample40 from Texas and sample41 from
Alberta, Canada, were includedto represent
gasesfrom carbonatereef environments.
They
show no indication of an abnormally high
(He/A)•,aratioasmightbe expected
frompure
carbonatesources.Sample41 contains12.2 per
the A4o/A
86 ratio as given by Wasserburg,cent H,S.
Czamanske,Faul, and Hayden [1957] and by
CARBON ISOTOPES
this paper (sample31, Table 2) showsa much
greater enrichment in radiogenic argon for
Isotopic analyseswere made on some of the
our analysis.This is most likely due to atmoscontainedin the gases.The
pheric contaminationin the older analysis.All carboncompounds
results
are
shown
in Table 3. The isotopic
of the Texas Panhandle gasesnow have similar
composition
of
the
carbon
is given in terms of
A4o/A
86 ratios, and this is believedto be a
regional characteristic.All of the high helium its delta ($) valuerelativeto the Chicagostandgaseshave (Ns/A,i,) ratiosof an orderof mag- ard, PDB [Craig, 1953].
Most of the Californiasamples
wererun only
as total gas;however,sincethey generallyconpossiblevalue of (A•ø/A•s),•dof 1.9 X 105. tainedlessthan 0.1 per centCOs,andverylittle
nitude or morehigherthan the atmosphericvalue.
Sample 22 was observedto have a minimum
This value is iarger than the maximum value
predicted by Gerling, Levskii, and Afanasyeva
[1956] by a factor of 2.4 and, therefore, adds
additional evidence[Wasserburg
and Bieri, 1958;
Signerand Nier, 1959]againstthe presenceof a
long-lived isomer of K 3s.
Samples 16, 17, 34, and 35 were chosen to
include carbondioxide gasesin the survey. The
Bueyerosfieldgases,whichare over 99.8 per cent
+•
total
'•'•-'•'^-' '
'
nearly equal to the methane carbon. Several of
the TexasPanhandleand the Keyes,Oklahoma,
highheli• gaseswerealsorun onlyastotal gas.
A comparison
wi•h samples29 and 30 from the
Texas Panhandle area, in w•ah both •tal
carbonand methanecarbonwere run, indicates
that about a 1-2 per •
differencemay be
expectedbetweentotal gas and methane carbon
CO,, showthe lowest(He/A)r,d ratios of any for this suite, with the methanehaving the
gasanalyzed.They alsocontainlessthan 0.5 ppm lighter carbon. The remainder of the natural
atmosphericargon, and have ds of 0.98-0.99. gaseswere analyzed for methane carbon, and
The Farnham Dome gasescontainapproximately in some casesan additional analysisof total
99.3 per cent CO,. They also have quite low
gas carbon was made. The carbon dioxide well
(He/A)r,d ratios and high values of •. Both gaseswereanalyzedfor CO•carbononly.Carbon
dioxidewasanalyzedfromall naturalgassamples
contatung more than 0.2 per cent CO• •th
these gas fields occur in areas containing carbonate rock and Tertiary basaltic intrusives
and lavas. Geologicevidence, as well as carbon
isotopic data which will be discussedlater in
this report, suggeststhat the COs was derived
from the decomposition
of carbonaterocksduring
metamorphism by the basalt. Under such
conditions,it is possiblethat most of the original
gases and liquids in the near-by sediments
were driven
off before the carbon dioxide
ac-
cumulation.This would explain the high purity
of the COs and the extremely low atmospheric
argon content of the gases.Since the molten
basalt might causea relatively higher releaseof
radiogenicargon,comparedto radiogenichelium,
than results from low temperature diffusion,
we could expect such an environment to yield
low valuesof (He/A),•d.
the exceptionof sample24. AH of the methane
analysesgive delta values between --57.6* and
--29.2 per mil. Both these extreme values are
from wells within the SacramentoValley gas
fields of California. Usually individual gasproducing dist•c•s possessa much narrower
range in methane carbon composition.This is
exemplified by samples 8-10 and 36-39 from
the Green River area and samples29-33 from
the Texas Panhandle. The methane carbon from
the Green River suite lies between --38.5 and
* Although this analysisfor sample 5 refers to
total gas carbon,it is from a gasessentiallyfree of
COs and other hydrocarbons;therefore, it undoubtedly is within a few tenths of a per mil of
the methane
value.
300
ZARTMAN,
WASSERBURG, AND REYNOLDS
--43.5 per mil, and the total carbonfrom the
contact metamorphism. The results from the
Texas Panhandle gases suggestsan even nar-
two CO•. fields studied in this report are in
agreementwith the work of Lang.
The isotopiccompositionof the oxygenfrom
rower
range.
These results are similar to the observations
made by Silverman and Epstein [1958] and three of the carbon dioxide wells was also
Wasserburg,
Gzamanske,
Faul, andHayden[1957], determined. Mitchell No. 4, Farnham Dome
who found that methane has the lightest carbon No. 2, and Farnham Dome Equity gave values
found in nature. The exact mechanismby which of (]co, relative to PDB of--23.0, --20.2, and
methane is producedis not known at present. --20.3 per rail respectively.Such values might
It is probablethat somebreakdownof organic be expected either for CO2 produced by the
matter, either by bacterialor inorganicprocesses, high temperature decomposition of marine
results in the liberation of methane. It was noted
carbonate or possibly by CO•. in equilibrium
by Silvermanand Epstein [1958] that natural with certain reservoir waters at appropriate
gas carbonis generallylighter than associated temperatures.In the first case,if fractionation
petroleum,and that this in turn is lighter than of only a few per mil occurredbetweenthe carthe organisms from which it is supposedly bonate and CO2 during such high temperature
derived. Rosenreidand Silverman [1959] have processesas contact metamorphism, we would
found an unusuallyhigh fractionationbetween obtain values of 5co• close to those of marine
methanol and the methane which is produced carbonate, i.e., --20 to --10 per cent [Clayton
from it by anaerobicbacterial decomposition. and Epstein, 1958; Engel, Clayton, and Epstein,
The methane is about 8 per cent enriched in 1958] recomputedrelative to the acid extracted
C • relative to the methanol. Further investiga- PDB standard. In the secondcase,we see that
tion of the origin of methaneis requiredbeforea although the (] value is too low to represent
morespecific
discussion
of its C•3/C• ratio can CO• in equilibrium with normal marine water
be made.
at room temperaturessuch values might result
An analysiswas made of a sample of gas fromequilibrationwith light watersor at elevated
collectedby Louis Gordonfrom a gas-seepover temperatures. [Epstein and Mayeda, 1953].
At least traces of carbon dioxide are observed
the head of the north fork of Scripp's Canyon
off the coast of southern California near La Jolla.
The delta value for this sample was --44.2;
it falls in the same general range as the well
methanes.I•. O. Emery (personalcommunication) considersthis gas to be a seep from a
subsurface source.
In thosegaseswhere total methaneand CO•
carbonwereanalyzed,it waspossibleto calculate
approximaterangesof the • value for the higher
hydrocarbons.
Becausesmalluncertainties
in the
known • values are greatly increasedin such a
calculation,it can only be concludedthat the
• value of the higher hydrocarbonsis generally
5-15 per mil greaterthan the methane.
If we consider the carbon dioxide gases, we
see that they are often associatedwith stratigraphic sections also containing carbonate
rock and igneousintrusives and lavas. Lang
[1959]foundthat the C•3/C•2 ratiosof several
CO2 gaseslay within the range of carbon in
marine limestones. This is in agreement with
the theory that the CO• comesfrom the nonequilibrium dissociationof carbonate during
in nearly all natural gases,and gasesranging in
compositionup to almost pure CO• have been
found. Someliterature [Lang,1959;Miller, 1937]
has been publishedon the origin of high carbon
dioxide gases;however, no detailed work has
been done on the CO•. in ordinary petroleum
gases.Here, the CO• generallymakesup between
a few hundredthsof a per cent and a few per
cent of the total gas.
The 5 valuesfor all the CO2analysesare seen
to lie between --21.9 and •-12.8. In every case,
the CO•. carbon is considerablyheavier than
that of the coexistingmethane or total gas.
This is a remarkableregularitywhich must have
important geneticsignificance.
If we assumethat the CH,-CO.,. pairs were in
isotopic equilibrium within a gas phase under
certain conditions,and that subsequentto that
time these values were 'quenchedin,' we may
calculateeffectivetemperatures.Such temperatures were computed using the fractionation
factor as calculated by Craig [1953]. These
results are given in Table 3. Since the kinetics
HELIUM,
ARGON, AND CARBON
of this exchange are completely unknown, we
are unable to say under what conditions we
would expect equilibrium to occur and be
frozen in. The possibleranges in temperature
which we might obtain, assumingequilibrium
to have occurredat sometime, are roomtemperature and the maximum temperaturewhich these
gasesmay have experienced.Some of the calculatedtemperaturesrangeup to severalhundred
degrees centigrade. This is considerably in
excessof the actual well temperatures. Since
it is generally agreed that petroleumsdo not
result through high temperature processesof
formation, we concludethat someof thesegases
lie well outside the range to be expectedfrom
equilibriumconsiderations.
Although it is thus doubtful that complete
isotopic equilibrium prevails, it is impossibleto
say whether the CIt4-COs pairs represent a
partial attainment of isotopic equilibrium. it is
significant that in every case of unreasonable
temperature,the t]co, value appearsto be lower
than the expected equilibrium value. That is,
if isotopic equilibrium were to be attained in
those gases, t]co. would have to become more
positive. Thus, assumingonly equilibrium processesto be operative between the COs and the
CH4, the observedt• values would representthe
maximumvalue whichthey couldhave originally
had. Although nine of the samples listed in
Table 3 have values of t•co. greater than --10
per mil and, therefore, may have possibly
arisenby either the equilibriumor disequilibrium
breakdownof marine carbonaterock, the other
seven samples for which we have data give
$ values too negative to representsuch a source.
If we assume that
all the carbon dioxides once
had $ values at least as negative as the lightest
sample now observed (•- --20 per mil) and
that the presentspreadresultedfrom later partial
equilibration, we must look for other sourcesof
carbon capable of yielding such values.
When COs occurs in much smaller concentra-
tions than CH•, any changetoward equilibrium
in the isotopiccompositionof the systemwould
causea muchgreaterchangein the C•a/C•sratio
of the COs than of the CHq. Thus, while such a
process
will affectthe original$c•, value only
weakly, even partial attainment of equilibrium
will tend to obscureany original value of •co,.
At present,it is not possibleto explainuniquely
301
the genesis of the carbon dioxide found in
petroleums.
CONCLUSIONS
All the natural gasesstudied exhibit values of
R which indicate that they are of a common
family and have obtained their radiogenicgases
from rather averagerock types. The variations
in abundanceof radiogenichelium and argon
in thesegasesare due to the effectsof leakage
and entrapment, solubility, porosity, and age
of the source rocks. Major differencesin the
abundancesof He cannotbe due to large variations (over a factor of 50) in the abundanceof U
and Th in the source rocks.
For many casesstudied,only a smallfractionof
the N,. presentmay be attributedto the incorporation of air.
The methane was found to have extremely
light carbon as suggestedby previousworkers.
In addition,it wasfoundthat the carbonisotopic
compositionof COs was always heavier than
in the coexistingCHq. This could not be reasonably attributed to completeisotopicequilibrium
betweenthesegaseousspecies.
It is possible that the concentration of atmosphericargon in a gas reservoir may be
correlatable
with the amount of reservoir water
with which it was equilibrated.
Acknowledgments.This work could not have
been carried out without the generousassistanceof
the many peoplewho aidedus in obtainingsamples.
In particular, we would like to thank the members
of the following organizationswhich supplied us
with material:
The U.S.
Bureau of Mines Helium
Activity Station, The Mountain Fuel Supply Co.,
Skelly Oil Company, Carbonic ChemicalsCorporation, Carbon Dioxide and Chemical Company,
Humble Oil and RefiningCompany,United Natural
Gas Company, Iroquois Gas Corporation, The
Sylvania Corporation, Standard of California and
affliates, and The Ohio Oil Companyand affiliates,
and Continental Oil Company and affiliates.
We would like to thank Dr. S. Epstein for granting us the liberal use of his laboratory and for his
continuedinterest in this problem. We also wish to
thank
Dr.
Sol Silverman
and Dr.
K.
Chave
who
contributed severalvaluable suggestions.Mr. C. W.
Mink assistedin the mass spectrometricwork at
Berkeley.
Contribution
No. 978 of the California Institute
of Technologywas supportedby a National Science
Foundation Grant, and in part by a grant in aid
from the CaliforniaResearchCorporationand by a
grant from the Atomic Energy Commission.
302
ZARTMAN,
WASSERBURG, AND REYNOLDS
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bility of nitrogen and argon in sea water, or.Phys.
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radiogenicA •s in potassium minerals, Geochim.
et Cosmochim.
Acta, 16, 302-303, 1959.
Silverman, S. R., and S. Epstein, Carbon isotopic
compositionof petroleums and other sedimentary
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Washington, 484 pp. 1959.
Urey, H. C., H. Lowenstam,S. Epstein, and C. R.
McKinney, Measurements of paleotemperatures,
Geol.Soc.Am. Bull., 62, 399-416, 1951.
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R. J. Hayden, Nat. Acad. Sci., Nat. Res. Council
Pub. 572, Nuclear ScienceSeriesReport No. 23,
as potassium and uranium. For this reason,
various carbonates of known uranium
content
were analyzed for potassium.The measurements
were made with a Perkin-Elmer flame photometer. Samples of the carbonatesweighing
1.02 gram weredissolvedin HC1 and evaporated
to dryness.They were put into aqueoussolution
with i ml of 12 N HC1 and diluted to 250 ml
volume. The solutions were• run on the flame
photometer and the readings compared with
those obtained on identically prepared CaC03
solutions of known potassium content. The
results were highly reproducible,and at concentrationsabove100ppm the data are probably
accurate to within 10 per cent of the values
reported.The data are givenin Table 6. Column2
of this table givesthe per cent of the CaCOathat
is aragonite.
Samples of the Bikini a•oll cores had been
analyzedpreviouslyfor uranium by Dr. M. S.
Coopsof the Radiation Laboratory, Livermore,
California. The analyseswere done using a
neutron activation technique in which the
Np TMwas counted.Dr. Coopskindly provided
us with thesesamples.
Someof the uranium analysesare the results
reportedby Tatsumoto
andGoldberg
[1959]usinga
colorimetricmethod. Dr. Goldberggenerously
providedus with someof thesesamples,and in
addition made other uraniumanalysesreported
here. Two samples of marbles analyzed for'
uranium by Dr. B. Doe are also included.
The analytical method used in each case is
Wasserburg,G. J., and R. Bieri, The A 3scontent given in the table.
of two potassiumminerals,• Geochim.et Cosmo- It is evident that in the carbonate rocks
156-158, 1957.
chim. Acta, I5, 157-159, 1958.
Wasserburg,G. J., and R. J. Hayden, A40-K40dating, Geochim.
et Cosmichim.Acta, 7, 51-60, 1955.
Wasserburg,G. J., R. J. Hayden, and K. J. Jensen,
A4ø-K•ø dating of igneousrocks and sediments,
Geochim.et Cosmochim.
Acta, 10, 153-165, 1956.
Wetherill, G. W., Variations in the isotopic abun-
reportedhere the potassiumcontentis widely
variable. The Muschel Kalk had the highest
value of 4,600 ppm and the Algal ls. and Leadville ls. the lowest values, 6 ppm. The high
potassium contents seem to be commonly
associated with sizable insoluble residues. The
304
ZARTMAN,
WASSERBURG, AND REYNOLDS
marblesGMW-2 and F6-B are rich in phlogopite.
It was found that the dissolutionprocedurewas
vigorousenoughto dissolveabout one-halfthis
mica. The values reported for samples with
potassic noncarbonatefractions will therefore
tend to be intermediate
between the values of
the whole rock and the pure carbonatefraction.
The weight per cent of insoluble residuesis
given in column 3.
contentsincethe uranium valuesremain roughly
the same for these materials.
The marbles show a range in K content
similar to that
found in limestones. All of the
marbles contained some dolomite. Sample
GMW-2 contains phlogopite, the larger part
being in the +80 sieve fraction. This effect is
clearlyseenin the resultson the total rock and
the --40 q-80 sievefraction. Of the two Franklin
The K/U ratio in purelimestones
suchas the marbles,F6-B containssomephlogopite.Sample
Spergen fm. and the Leadville Is. may be as
low as 25. This ratio is lower than that in average
igneous rocks by a factor of 400, and would
yield a value of R of 3 X l0 s. This differenceis
due to the great decreasein the potassium
F6-A
was taken
in the mine and is from
a
lithology which contains some coarse biotite
crystals.No biotite was observedin the sample
analyzed.
Sample fd 7-3 is a white dolomitic marble
TABLE 6. Analytical Data of Potassiumand Uranium in Carbonates
%
Samples
Wgt. %
Aragonite
Residue
ppm K
ppm U
59
10.2
321
2220
0
1.4
198
1.082
Dol.
46
0
0.7
0.6
131
112
6
0.012
0.872
1.412
K/U
LIMESTONES
Buckhorn
Buckhorn
residue
Cretaceous Chalk
ChicagoFormation
BasalCoquina
Pr 2--Algal Isl.
Muschel Kalk
Muschel residue
0
Oolites(0-03)
GMW-2 (-40q-80), •narble
GMW-2, total hand
spec.crushedmarble
17.1
4600
2900
98
0. !
87
0
4.1
1000
7.1
2100
0
fd 7-3, marble
fd 7-2, marble
4,3.2
< 0.3
0
F6-B Franklin
0
1.6
384
0
0
0.4
0.4
58
6
Leadville Limestone 32
Leadville Limestone 82
Leadville
Limestone
40
3.4'
< 60
390
0.122
333
0.282
0.242
207
25
0
0.3
20
0
0.4
25
1.02•
Calcite I
Calcite II
0
0
<0.1
<0.1
_<5
_<5
<0.01 •
0
<0.1
9
Calcite III
25.6
0.0834
0.6404
SpergenFormation
MODERN
128
<0.1
-<5
235
F6-A Franklin Marble
Marble
188
13100
147
4.25
24.5
SiII,;LLS
1336
1337
99
100
1338
100
1339
1345
1348
100
99
100
s-07 (B)
100
S-11
S-13
S-14
S-15
S-17
100
99
100
< 0.1
< o. 1
<0.1
1t}2
74
38
54
79
35
153
0.3
0.4
0.6
62
51
39
44
154
0.013'
0.012'
0.067
0.022
0.032
0.041
•
•
•
•
11.77
5.17
0.76
1.77
1.38
3.76
X
X
X
X
X
X
10 a
l0 s
l0 s
10 •
l0 s
10 •
HELIUM,
ARGON, AND CARBON
305
containing65 per cent dolomite and 35 per cent
diopside.The residueof this samplewas found
to contain 61 ppm of potassium.Samplefd 7-2
is a dolomitic marble containing 0.26 per cent
graphite. These samples were given to us by
Dr. B. Doe, who determined the uranium
contents in the courseof another investigation.
Samplesof calcitesparwererun for comparison.
It is evident that thesecarbonateshave extremely
low levels of potassiumas suggestedby A. E.
Engel (personalcommunication).
In order to determinewhether any potassium
is present in the original shell materials which
inorganically precipitated CaCO•, such as the
calcitespar,hasvirtually no potassium;however,
the oolite samplegave a result comparablewith
the shells and corals. These data suggestthat
carbonatesprecipitatedfrom seawa•er will have
somepotassiumin chemicalcombination. On the
other hand, it is possiblethat some potassic
clays may be containedin all theseprecipitates.
Somewell preservedfossilshellswere analyzed
for K. SamplesI and II were large complete
specimens which were carefully cleaned to
eliminate any contamination. Sample III was
taken from a glauconite-rich matrix and a
constitute
small amount of contamination
limestones
as
distinct
from
con-
with this mineral
tributions from clay materials and detrital
particles, analyses were made of modem and
could easily cause this high result, while it
appears that under optimal circumstancesthe
fossil shells and corals. The concentration of
potassiumcontentof shellsmay be preservedin
potassiumin both modem shellsand corals are fossils.The concentrationlevels are very low
roughlythe sameand rangefrom 30 to 150 ppm. but it may be possibleto utilize these materials
It follows that these materials contain a signifi- for A4ø-K4ødating with the most modern techcant amount
of this element when formed.
niques. Assumingcomplete retention for sample
Many of the purer limestoneshave K con- I, this wouldcorrespond
to 2 X 10-s cc STP/g
centrations lying inside of this range. Some of A•ø. While this is readily measu.rable,the
TABLE
Samples
FOSSIL
%
Aragonite
99
100
III Shell Fragments
MODERN
Wgt. %
Residue
ppm K
0.2
0.3
29
27
ppm U
K/U
99
0.7
20i
70
99
0.3
31
50
2.5 •
3.2'
12.4
15.6
2.3-3.23
4.2-5.2
2.2-3.2
6.5-8.1
12.5
11.6
4.7-5.7
3.1-4.0
15.7
12.4
4.1-4.9
3.3-4.13
4.3-5.1
3.4-4.2
6.1-7.5
12.4
14.3
25.0
6.3
CORAL
Coral I (0-04)
Coral II (0-05)
Corals:
Tare, depth 4'
51
0.2
35
13'
49
0.3
43
34'
63'
89
73
0.1
0.2
60
65
90-1/2'
66
0.1
85
97'
51
0.6
63
66
0.5
46
62
62
97
0.5
0.3
0.2
67
96
43
Roger, depth 4'
8'
23'
44'
FOSSIL
Continued
SHELLS
I
II
Bikini
6.
8.2
CORAL
1437 Reef Coral
99
63
• M. Tatsumoto and E. D. Goldberg(Colorimetric) Geochim.et Cosmochim.
Acta, 17, 201, 1959.
"B. O. Goldberg(Fluorescence)personalcommunication.
8 M. Coops(Neutron activation) personalcommunication.
• B. Doe (Isotope dilution) Thesis,Calif. Inst. of Technology,1960.
300
ZARTMAN, WASSERBURG, AND REYNOLDS
problemof correctionfor atmosphericargonwould found to be from 400 to 3000 ppm. This is a
muchhigherlevel than foundin shells.If it is
be formidable.
The shellsappear to have a uranium concen- assumedthat this potassiumis in the apatite
tration about 100 times smaller than the corals. crystalsrather than in fluids in the marrow,
fossil
The K/U ratiosfor the shellsare muchcloser it may be possibleto date well-preserved
to the values for averageigneousrocksthan the
bone of Cretaceousage.
corals which have ratios 1000 times smaller.
Acknowledgments.We would like to thank Dr.
H. A. Lowenstam for providing us with many of
the samplesof limestonesand shells.The authors
The
ashed
bones
of
15
different
modern
vertebrates were also analyzed to investigate are indebted to Mr. Theodore Wen and Mrs.
the possibility of dating appropriate fossil Dorothy Settle for their careful work on the potasmaterials. The total range in concentrationwas sium determinations.
TABLE 7. Descriptive Data
LIMESTONES
Buckhorn,Mid-Pennsylvanian,Sulphur, Okla.
Cretaceous
Chalk, Campanian,Vigny, France
ChicagoFormation,Niagaran, reef core, Thornton, Ill.
Basal Coquina,DevonshireMarine Isl., Grape Bay, Bermuda
Pr œ,Mid-Eocene, Algal Isl., Puerto Rico
MuschelKalk, Mid-Triassic,Rorigliana,Italy
Oolites(0-03) BahamaBank about 1 mile S.E. of S. Pt. Cat Cay 79ø15'W,25ø30.5'N water depth3 feet
(recent)
GMW-œ (-•0-•80 Mesh Fraction) Marble, Balducciquarry, Gouverneur,N.Y.
GMW-œ, Marble, totalhandspec.,Balducciquarry, Gouverneur,N.Y.
Fd7-3 Marble, fetid limestone,shoreof Sylvia Lake, N.Y.
Fd7-œMarble, graphitic,900 level, shaft of No. 3 mine, Balmat, N.Y.
F-6A Marble, New JerseyZinc Mine, Franklin, N.J.
F-6-B, Franklin Marble, Franklin, N.J.
LV-32,
Elk
Creek
Colo.,
zone
i t
These
samples
are
de-
L V-82, BaritesCabin,Colo.,zone2
LeadvilleLimestone
scribedby Engeland
LV-128, SweetWater Lake, Colo.,z.onei
Engle.Ms. in preparation
SpergenFormation,Ooliticportionof the SpergenFormation,Mississippian,
St. Genevieve,Missouri
CalciteI, Iceland Spar
CalciteII, Manhatten, Nev.
CalciteIII, Spar 611, Hockerville, Okla.
SHELLS
1336 Lodakia Orbicularis,Belmont Isl., QueensFree Cave, Ferry Rd., Bermuda
1337 LaevicardiumLaevigatum (recent) (TM•), Patorreef Lagoon,Bermuda
1338 Lodakia Orbicularis(recent), WhaleboneBay, Bermuda
1339 LaevicardiumLaevigatum(Pleist.), BasalDevonshire,Marshall Isl., Bermuda
1345 ConusCalifornianus(recent),Naval BaseE. SantaBarbara, Calif.
1348 ConusCalifornianus(Pleist.), Hilltop Quarry, San Pedro,Calif.
S-07 (B) Haliotis Corrugata,La Jolla, Calif.
S-11 Littorine Planaxis,La Jolla, Calif.
S-13 AcmecaLimatula, La Jolla, Calif.
S-1• Olivella Biplicata, La Jolla, Calif.
S-15 Acanthina Spirata, La Jolla, Calif.
S-17 Tetraclita squamose,La Jolla, Calif.
FOSSIL SHELLS
FossilI Crassatelites
Vadosus,L. Maestrichtian,Ripky Formation,Coon Creek, Tenn.
FossilII Trigonia Stantoni, CoonCreek, Tenn.
III ShellFragmentsMathewsLandingMarl., Paleocene,WilcoxCo., Ala.
CORAL
Coral I (0-04) Key Largo Is. (Diploria labysinthiformis)
CoralII (0-05) Key Largo Is. (Montastrea Annulasis)
Bikini Coral taken at different depths
1]•37Ree! Coral Pleistocene,Bermuda
(ManuscriptreceivedApril 12, 1960;revisedOctober11, 1960.)